Two of the letters were sent to Viacom, addressed to Jon Stewart and Stephen Colbert. (Getty Images)

(Newser)
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A mystery letter-writer is threatening to decimate Congress with biological agents. Letters, some of them containing a powdery substance found to be harmless, have been intercepted at the offices of several members of Congress, and at media organizations, including the Washington Post and Viacom, which received two letters addressed to Jon Stewart and Stephen Colbert. "There is a 10% chance you have just been exposed to a lethal pathogen," warn the letters, according to AP, which obtained a copy of one.

The letters, mailed from Portland, Oregon, were signed: "The MIB," and bore a phony return address. The sender calls for "an end to corporate money and lobbying," and an end to corporate "personhood." Lawmakers and their aides have been warned to be on the alert. "So far, none of the letters have contained a hazardous substance," an FBI special agent says. "We are working with those law enforcement agencies affected to determine if the mailings are related. We take these matters seriously, and will investigate fully."